Selected area hardcoating of aluminum



United States Patent 3,367,852 SELECTED AREA HARDCOATTNG 0F ALUMINUM James F. McGivern, Jr., Avon, Conn, assignor to United Aircraft Corporation, East Hartford, Conn, a corporation of Delaware No Drawing. Filed Oct. 29, 1964, Ser. No. 407,568

7 Claims. (Cl. 204-15) ABSTRACT OF THE DISCLQSURE In a manufacturing operation wherein a thick, abrasionresistant coating of aluminum oxide is to be provided on selected areas of an aluminum article in a hardcoating process, the article is first anodized in a chromic acid bath to a sel-fwsustaining voltage. The thin anodized coating thus provided is then stripped away to expose the basis metal in those selected areas where the thin coating is desired, and the article is subsequently hardcoated, the thin anodized coating acting as a maskant in the hardcoating operation.

This invention relates in general to the production of hard, abrasion-resistant coatings on aluminum and aluminum alloys and more particularly to an improved method of surface hardening .selected areas of aluminum and aluminum alloys.

As used throughout the following description it will be understood that, where the word aluminum is used, it includes both the pure metal and those alloys of aluminum containing in general less than weight percent of the heavy metals and less than 5 percent copper or 8 percent silicon.

The term hardcoating as used throughout will be understood to have reference to, in context, either the method of producing thick anodic oxide coatings on aluminum by long treatment at higher than normal current densities in a low and closely controlled temperature range, or the coatings produced by said method. For a more detailed description of hardcoating techniques reference may be made to the US. patents to Burrows, Nos. 2,692,851 and 2,692,852.

In the production of components fabricated from aluminum it is well known that selected area protection must be imparted thereto in certain applications to reduce the tendency of the aluminum to gall upon assembly or to diffusion bond to surfaces with which it is in intimate contact. Aluminum is known to be inherently soft and, consequently, when used in a bearing application, has been demonstrated to have very poor abrasion resistance. An aluminum surface in the absence of special treatment will exhibit severe galling and ultimate seizure upon continued rubbing contact.

Aluminum, although it is one of the most active of metals, rapidly forms a thin, adherent and protective oxide film upon exposure to air which, although it will minimize further normal surface oxidation, is generally unsatisfactory as a bearing surface. The naturally-occurring oxide film is not of sufiicient thickness to substantially alter the wear characteristics of the metal substrate. However, aluminum oxide is known to be one of the hardest of minerals and, by electrochemical means, an oxide layer may be formed on the metal surface which is of sufficient thickness and adherence to form such a suitable bearing surface. But aluminum oxide, as is the case with most ceramic materials, is inherently brittle and its use is typically held to a minimum in apparatus fabricated to close tolerance. Further, it is virtually impossible to form a satisfactory uniform coating of considerable thickice ness around a sharp edge. It is generally preferable, therefore, to form the hardcoating only on those metal surfaces Which will be actually exposed to the abrasive action of cooperating parts.

The conventional method of selected area hardcoating involves masking of those areas which are not to be hardcoated with a protective laquer which is stable and nonreactive with the hardcoating solution. The protective lacquer chemically isolates the surfaces to which it is applied from the oxidizing effects of the hardcoating solution and, therefore, permits formation of the thick oxide layer only in those areas unprotected by lacquer. Application of the lacquer has been demonstrated to be a time-consuming and demanding job, particularly with complex components, and the advantages of eliminating this heretofore necessary step in a production line are evident.

The production of such components typically follows a sequence such as that which follows: an aluminum blank is machined to its correct overall dimensions except in the selected areas where hardcoating is to be subsequently effected, these areas being only rough machined; the semifinished part is anodized completely, by any of the well-known methods, forming an oxide layer of a typical .00004 to .0001 inch thickness; the part is then finish machined exposing in the process the base metal in those above-mentioned selected areas; a lacquer maskant is manually applied to the part except where the base metal is exposed; a hardcoat is applied to the base metal areas; and the maskant is removed by chemical means.

Aluminum oxide is well known as an electrical insulator. However, an anodized coating as conventionally applied is structurally inadequate to, of itself, satisfactorily perform the desired masking function, hence the need for the lacquer maskant. In the trade in general, chromic acid anodizing is accomplished at a current density between 1-5 amperes per square foot on a power supply limited to volts D.C. Usually the operation is conducted on the basis of a uniform voltage increase up to the 40 volt limitation. It has been demonstrated, however, that anodized coatings thus applied will not inhibit further oxide formation in a hardcoating process and are unsuitable as maskants in this environment.

In the anodization of aluminum, utilizing chromic acid, a thin impervious oxide film is first formed followed by the formation to a greater thickness of a porous outer layer. This porous outer layer is characterized by a pore or cell structure consisting of columns of oxide and interspaced pores extending normal to the metal surface. The conditions of a dry dielectric test of this configuration,

reflecting by nature the electrical resistance from the top of the columnar structure, always results in a higher dielectric strength than that normally observed during anodization. During anodization, the electrolyte penetrates the permeable layer and registers an effective electrical resistance only with respect to the thin, inner impervious layer. Tests conducted on conventional anodic coatings, with or without pore sealing, in. a hard coating solution at normal hard coating voltages are characterized by successive perforation failures in the anodized layer leading to a complete coating breakdown in periods as short as one minute. The anodized coating voltage breakdown thus resembles the perforation failure of dielectric material subjected to voltage gradients in excess of their capability to function as insulators.

It is, therefore, an object of this invention to provide an anodic coating which will satisfactorily operate as a maskant in a hardcoating process.

It is a further object of this invention to provide an improved method for the hardcoating of selected areas of aluminum articles.

An additional object is to eliminate the hand-masking step in the production of hardcoated aluminum components.

These and other objects of this invention will be specifically pointed out or will be obvious from the following description and should not be construed as limiting the scope of this invention.

In the light of observations regarding the mode of anodic coating breakdown, it was evident that the lack of coating uniformity and perfection was almost totally responsible for the failure of this coating to perform satisfactorily as a hard coat maskant. When parts are processed on a voltage program to 40 volts and then maintained for periods at that voltage, the current declines. It was felt, therefore, that if the current was held constant, the voltage of the chromic acid anodic layer formation could be made to exceed 40 volts. As a matter of fact, a current density in the range of 1-12 amperes per square foot will produce self-maintained overall voltages in the range of 110130 volts which exceeds the minimum breakdown voltage of about 90 volts required of the coating during the subsequent hardcoating operation. This condition of self-maintained voltage will usually be achieved within the period of 30-120 minutes, the particular time in a given case depending on a number of factors including the characteristics and condition of the anodizing bath, the amperage level selected, and the particular alloy being anodized. In a constant current anodization process the parts being coated experience an immediate voltage rise followed by a more gradual voltage increase to the steady state voltage mentioned above. The voltage program effected in this manner is roughly exponential and its slope increases with increasing current density.

In an anodizing process While electric current flows in the system, oxides of aluminum will form at the expense of the parent metal. As these grow in thickness and uniformity, the effective electrical resistance of the coating will similarly increase. For any given current density, therefore, the voltage rise is approximately proportional to the anodic coating thickness and continuity. At some elevated voltage, however, the anodic coating tends to perforate in the manner of a dielectric perforation failure. Because the surrounding material has a higher electrical resistance, the current path is concentrated in the perforations. These then reanodize quickly and perforations occur in other localities. With constant current anodization, however, the perforations gradually become fewer in number and the consequent anodic coating becomes generally more uniform and resistant to perforation. The improved coatings thus formed at the elevated voltages are less prone to failure at the voltages used in the hardcoating process and perforations which do form tend to be self-healing. This is precisely the anodic coating condition required for a maskant of this type for use in selected area hardcoating.

The ease of reduction of the number and severity of perforations in anodic coatings is dependent to some extent on the metallurgical and chemical characteristics of the substrate. As has been indicated, satisfactory coatings cannot be achieved, in the absence of special techniques, where the aluminum alloy contains more than 5 weight percent of the heavy metals or copper or more than 8 percent silicon. The normally accepted range of compatibility of the various alloying elements are Well known in the trade. Similarly, anodized coatings produced by other methods utilizing sulphuric, oxalic or phosphoric acid baths do not appear to yield a film suited to a rnaskant function. Alloy grain size is introduced as a control function because of the varying crystal areas to be continuously covered by the anodic coating, the probability of complete coverage increasing with decreasing grain size. The relative grain size effect can be controlled to a limited degree by manipulation of the anodizing bath temperature. For example, wrought alloys are anodized at temperatures up to 94 F. while castings are anodized at temperatures as low as 60 F. The temperature variation extends the ability of the alloy to accommodate the oxide on the same growth plane or provides for accommodation by better matching of other growth plane systems. In any event, it is observed that coarse grained materials such 'as castings are most successfully processed at bath temperatures lower than those satisfactory for fine grain material such as wrought alloys.

In order to illustrate more clearly the nature of the present invention, the following example of a typical selective area hardcoating procedure is set forth, it being understood that this example is presented for illustrative purposes only and not as limiting the scope of this invention:

Example A large number or" aluminum alloy parts were machined to finished dimensions except in the areas to be subsequently hardcoated, these areas being rough machined.

Each part thus machined was connected as the anode in a chromic acid anodizing bath consisting of essentially a 3 weight percent aqueous solution of chromic oxide held at 60-94 F.

A constant current of 1-12 amperes per square foot, most preferably 35 amperes per square foot, was imposed on the system and held for one hour. In this regard, it is to be understood that voltage programming, While unnecessary, is not detrimental, providing the upper voltage is allowed to attain a self-maintaining voltage condition.

The anodized parts were then finish machined, a bare metal surface thus being exposed at the areas to be hardcoated.

The parts thus prepared were immersed in a hardcoating bath consisting of an aqueous solution of sulphuric acid, said acid comprising l0-25 weight percent of the solution, and preferably 15 weight percent, saturated with carbon dioxide and held at 15 -25 F.

A constant current of 10-25 amperes per square foot, preferably 15 amps per square foot, was imposed on the system and held for one hour.

Although it has been demonstrated as unnecessary for the most part, sealing of the thin anodized layer is usually effected with scalding demineralized water or with nickel acetate solution of 5.35.8 pH at -295 F. The sealing step is generally undertaken to further enhance the reliability of the basic coating. In effect, it supplements the coating thickness at the base of the pores in the pervious outer layer.

The resulting articles exhibited an anodic coating of from 0.04 to 0.1 rnil and a hardcoat thickness of 2 mils. The hardcoating thus applied could be regulated precisely as to thickness and hardness and was confined to the areas from which the anodizing had previously been mechanically stripped.

The constant current anodization masking process and hardcoating procedure of this invention has been demonstrated to be entirely reliable in application. The process is highly insensitive to a variation of process variables such as current density. It is therefore possible to proces components under less stringent control conditions than heretofore possible, and to process wrought and cast alloys, and parts made from different alloys simultaneously. The coatings themselves are far superior to conventional coatings with respect to the well-understood characteristics of coatings. Further, there is no sacrifice of anodic film thinness affecting overall dimensional re quirements.

By this process, a hard coat may be generated on selected metal areas with very precise dimensional controls, permitting thereby, very close control of tolerances in close fitting parts. Further, because of the reproducibility of such coatings with respect to hardness, the wear life of the bearing surfaces thus produced may be accurately forecast.

While a specific example has been shown in the way of illustration, it Will be apparent that many changes and modifications in the process and techniques set forth may be made without departing from the spirit of this invention and within the scope of the following claims.

I claim:

1. The method of selective area hardcoating of aluminum and aluminum alloy articles comprising anodizing said articles in a chromic acid bath at a current density of 1-12 amperes per square foot to a self-sustaining voltage to a coating thickness of at least 0.00004 of an inch, stripping said coating from selected areas of said articles, and hardcoating said articles to form a hardcoat of at least 0.002 of an inch on said selected areas.

2. The method of claim 1 wherein said chromic acid bath consists of essentially a 3 weight percent aqueous solution of chromic acid and in which the current density utilized will result in a self-sustained voltage of at least 90 volts.

3. The method of claim 2 wherein said hardcoating bath consists of essentially 25 weight percent of sulphuric acid.

4. The method of selective area hardcoating of aluminum and aluminum alloy articles comprising the steps of:

(a) electrically connecting said articles as the anode in a chromic acid anodizing bath,

(b) applying a constant current through said anode of 1-12 amperes per square foot to a self-sustaining voltage to an oxide film thickness of at least 0.00004 of an inch,

(c) mechanically stripping said film from selected areas of said articles,

(d) and hardcoating said articles.

5. The method of claim 4 wherein a hardcoat layer of at least 0.002 of an inch is formed.

6. The method of selective area hardcoating of an aluminum alloy article comprising the steps of:

(a) electrically connecting said article as the anode in a chromic acid anodizing bath,

(b) anodizing said article to a self-maintaining voltage of at least 90 volts at a bath temperature of 94 F. for 30-120 minutes,

(c) mechanically stripping the oxide film produced from selected areas of said article,

(d) and hardcoating said article in a hardcoating bath at 15 40 F. to a hardcoat layer thickness of at least 0.002 inch.

7. The method of claim 6 wherein the hardcoating bath consists of essentially 10-25 percent by weight of sulphuric acid held at a temperature of 15-25 F.

References Cited UNITED STATES PATENTS 2,691,627 10/1954 Johnson 204l8 2,692,851 10/1954 Burrows 204-58 2,993,847 7/1961 Poch 204-15 OTHER REFERENCES Mozley, Paul F.: The Chromic Acid Anodizing Process Metal Finishing, June 1941, copy in 20458.

HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

T. TUFARIELLO, Assistant Examiner. 

